UN aid officials have warned that the situation in the Syrian town of Daraya is extremely dire, with shortages of food, medicine, and clean water.

At least 4,000 people are besieged in the town by Syrian government forces.

Speaking in Geneva, UN emergency relief coordinator Stephen O'Brien said the Syrian government had ignored "countless" requests for aid to be allowed in.

Residents of the town last received a delivery of aid in November 2012.

UN officials were able to make a needs assessment earlier this month, and found a severe lack of food and medicine and near complete destruction of the water supply, leading to shortages of safe drinking water.

The town's electricity supply was cut off more than three years ago.

"We will continue to press the Syrian authorities relentlessly for us to have that safe unimpeded access to Daraya," Mr O'Brien said.

The UN says Syria's shaky cessation of hostilities has allowed significant aid to 12 of 18 other besieged areas, the most recent being this week to the northern town of Rastan, whose 120,000 people had received no aid for over a year.

Almost half a million people live under siege in Syria, the UN estimates. Last week, relief agencies evacuated 500 wounded people from four besieged Syrian towns, in what has been described as the largest such operation so far in the five-year conflict.

Half were brought out from towns blockaded by pro-government forces, and half from towns blocked off by rebels.

Meanwhile, peace talks that restarted in Geneva earlier this month are in difficulty. It is not clear whether a partial truce that has so far lasted eight weeks will hold.

Opposition representatives walked out of the talks last week, blaming government violations of the ceasefire, foreign media reports.